Jetting questions (not a GS, but close)

[t3rmin]

Since you've tried both position 3 and 4 from the top with (if I remember) lean and then rich symptoms respectively, then the most logical thought would be to use a jetting spacer (typically .022" thick) to achieve position "3 1/2". Place the e-clip at position 4 from the top with the jetting spacer directly on top the clip. Maybe that will fix things.
If that doesn't work then the air jet could factor in here. I believe those carbs have a pilot air jet and a primary air jet. Pods many times require air jet changes too.
Another thought would be to change the jet needle size or the needle jet size. You're getting in a bit deeper there. Needle jet change isn't normally needed with your mods. Jet needles often need changing. Heck, you could even have worn needle(s) or needle jets causing a problem.
I'd try position 3 1/2 first.

I think you're right. I predict position 3.5 will be closer, and if I want to be right-on, I'll have to get a differently-tapered needle (that would seem to be the way to separately-tune the needle's top and bottom). Thanks.

 

[KEITH KRAUSE]

And I assume the carbs are completely clean.
If the old Uni's were rotting, the dinky little foam pieces will jam into the air jets and pilot circuit passages easily. After dipping/spraying, I shove at least 150 PSI through everything and verify it comes out the other end of passage.
And did you mention vacuum synching? Has to be done to accurately judge performance.

 

[KEITH KRAUSE]

OK I gotta go elsewhere for now.
Good luck with it and I'll try to check in later and help if I can.

 

[t3rmin]

Yeah they've been cleaned thoroughly. Vacuum synched as well. Yamaha put springs on the sync screws instead of locknuts like Suzuki. Sync in a fraction of the time, and with only a screwdriver. \\/

 

[t3rmin]

Also I believe the air jet is pressed in, so changing that isn't really an option.

The more I think about this, the more I think needle taper is what I'm fighting against. (Or needle wear, but I've examined them pretty close and they don't *look* worn.)

Here's one of 'em. They're 5GZ6

I wonder if something with a more linear taper would work better?

 

[t3rmin]

Ok, now I'm glad I messed with the float levels. I realized I hadn't tried position #3 with the 137.5 jets since drilling the slides, so I tried it. This time the low end didn't disappear, due probably to the richened float levels. This is the best it's run so far. I can still cause it to go rich in the middle if I roll-on fast, but with carefully-metered twists of the wrist, I can go through several gears, pulling hard all the way.

I'll try position #2 tomorrow, and if that's too lean, shim it to 2.5. But I'm going to have to call it a night now, since these test rides at 29 degrees are freezing me to death. ;-)

 

[LeeGS550E]

I'm following this with interest. You're doing a very good job, Matt! This thread's a nice example of the sceintific carb-tuning process at work. I'll bring the 5/7 up so you can get it dialed in next.:-D

 

[Guest]

You might have to tune it again when it gets warm. :?

 

[t3rmin]

You might have to tune it again when it gets warm. :?

Pffft! :-&

 

[t3rmin]

I'm following this with interest. You're doing a very good job, Matt! This thread's a nice example of the sceintific carb-tuning process at work. I'll bring the 5/7 up so you can get it dialed in next.:-D

Hah. Actually I've made two changes at once a few times, following a hunch, which has almost always been a step backward, so I've been anything but scientific. I lack discipline...

But this thread has a been a huge help, not just the advice (which has been great, keep it coming!), but writing it all down and collecting my thoughts.

Also it's a big help not having an airbox to futz with when removing carbs, and the XS throttle cable is super easy to disconnect, and there's no choke cable (lever built-in to the carbs), and there's lots of room in that big bike frame, so they slide in and out super easy.

 

[t3rmin]

Update:
Moved needle clips to #2 from top (was on #3). Good news is middle/end is improved, and I can fairly easily avoid the sloppy area by keeping throttle under 3/4 between 4.75k and 6.25k (ideally a smooth roll-on, but that's the touchy area). I hypothesize position #1 would pretty much eliminate the soggy area, but reduce performance at all other revs.

Bad news is the flat spot off-idle is back, as I thought it would be. Not quite as bad as last time I tried position #2, probably due to the float adjustment, but it's quite annoying. Best off-idle performance was actually position #4. At #1 I'm sure you could barely get the bike moving from a stop.

Overall, I think position #3 is better, since I can actually get a good launch, and if I'm very careful to do smooth calculated roll-ons, power at all revs is a little better.

I could try #2.5 with a spacer, but I know what'll happen. Mid/top will have slightly more blubbery potential and low end will be a little better.

So my needles/slides come in too lean and go out too rich too soon. Or, stated the other way, I need the needle circuit to come in richer and go out leaner and/or later. Can't go too much later, redline is only 8.5k.

Possible fixes:
1) Different needle jets? No experience here -- do they enrichen more at a certain range or would this affect the entire needle circuit evenly? If it's even, this'd do me no good -- I've got to bias the needle circuit toward leaner at the end.

2) Different needle taper or longer needles? I'm pretty much lost here as well. Those Mikuni needle codes are confusing.

3) Retard slide action. Epoxy holes and drill to smaller size. I'm thinking even smaller than stock. This is the one I could try today without spending additional $$$. However, I'm not sure it would do more than mask the problem, and probably only in the lower gears. During a long run in 3rd or 4th or 5th the slide will have plenty of time to rise, regardless of the size of that orifice, right? So that'd end up being another balancing act between throttle response in the lower gears and keeping the slide SLOW enough to not get over-rich in the higher gears. The size of the slide hole is for more of a damping effect, right?

4) Stiffer slide spring? I just now thought of this one. Rather than change damping, this would actually change how MUCH vacuum is needed to get the slide to a particular position, right? So it would delay full slide opening to bigger throttle openings/higher revs?

Very eager to hear everyone's take on these thoughts!

 

[t3rmin]

Now that I think about it, I think I will have to try making the slide holes smaller. I remember when I drilled them the midrange thing got noticeably worse, but I just dismissed that as aggravating whatever else was the REAL problem...

 

[t3rmin]

Slide holes filled with JB Weld and drilled to about 1.5mm (I think stock was about 2mm), which is about half what they were (3mm). That should give me a good feel for the effect. Just waiting for some time to re-install and a break in the weather to test...

Note regular epoxy (at least the stuff I tried) is not very "drillable". Just kinda melted and yielded a very crappy hole. JB Weld (second try) drilled cleanly, so there is a difference.

 

[t3rmin]

Got the carbs back in the bike. I also stretched out the slide return springs a bit, which increased the free length somewhat. I'm not sure if that'll make any permanent difference, but I've heard it can be helpful.

Anyway, I took it out for a test ride and dumped it. Was too wet to get a good feel for the changes, but the good samaritan who drove it home for me (clutchless) said it ran great. ;-) (see thread in off-topic)